In an etching process, a deposition process, an oxidation process, or a sputtering process for manufacturing a semiconductor device or a FPD (Flat Panel Display), there has been used plasma in order to make an effective reaction of a processing gas at a relatively low temperature. In such a plasma process, there has been used a high frequency power (RF) or a microwave in order to discharge or ionize a processing gas within a vacuum processing chamber.
By way of example, in a capacitively coupled plasma processing apparatus, an upper electrode and a lower electrode are arranged in parallel with each other within a processing chamber. Further, a target substrate (a semiconductor wafer, a glass substrate or the like) is mounted on the lower electrode, and a high frequency power having a frequency (typically, about 13.56 MHz or higher) suitable for generating plasma is applied to the upper electrode or lower electrode. A high frequency electric field generated between the electrodes facing each other by applying the high frequency power accelerates electrons, and plasma is generated by a collision and ionization between the electrons and a processing gas. Further, by a gas phase reaction or a surface reaction of radicals or ions contained in the plasma, a thin film is deposited on the substrate, or a material or a thin film on a surface of the substrate is etched.
As described above, radicals and ions incident onto a substrate may be an important factor in a plasma process. In particular, the ions are important in that the ions have a physical action by an impact when the ions are incident onto the substrate.
Conventionally, in a plasma process, there has often used a RF bias method. In a RF bias method, a high frequency power having a relatively low frequency (typically, about 13.56 MHz or lower) is applied to a lower electrode for mounting thereon a substrate. Also, ions contained in plasma are accelerated by a negative bias voltage or a sheath voltage generated on the lower electrode, and are attracted to the substrate. In this way, the ions in the plasma are accelerated and the ions collide with a surface of the substrate, so that a surface reaction, an anisotropic etching process or a film modification (film reform) can be promoted.    Patent Document 1: Japanese Patent Laid-open Publication No. H7-302786
In a conventional plasma processing apparatus having the above-described RF bias function, only one kind (a single frequency) of a high frequency power for attracting ions applied to a lower electrode is used. Further, this high frequency power, and a self-bias voltage or a sheath voltage on the lower electrode are used as control parameters.
However, as a result of repeated experiments on an action of RF bias conducted by the present inventors during a development in a technology of a plasma process, it has been found that it is difficult to control an ion energy distribution in a high-tech plasma process of multiple process characteristics by a conventional method using a single high frequency power for attracting ions.
To be more specific, as a result of analyzing an energy distribution of ions (IED: Ion Energy Distribution) incident onto a substrate when a single high frequency power for attracting ions is used, as depicted in FIGS. 19A to 19C and 20A to 20C, energy of all incident ions is regularly distributed in a continuous energy band, and lots of ions are concentrated (peaks are shown) in a vicinity of the maximum energy and in a vicinity of the minimum energy. Therefore, if it is possible to readily vary not only an average value of ion energy but also the maximum energy and minimum energy on which the ions are concentrated, it is expected that a RF bias function required for a plasma process can be effectively controlled. However, it is deemed that this is not available.
In accordance with a conventional method, when a high frequency power having a relatively low frequency of, for example, about 0.8 MHz is used for attracting ions, if a RF power is varied, an ion energy distribution characteristic is varied as shown in FIG. 19A (low power), FIG. 19B (intermediate power), and FIG. 19C (high power). That is, while the minimum energy is fixed to about 0 eV, the maximum energy is varied into about 1000 eV (FIG. 19A), about 2000 eV (FIG. 19B), and about 3000 eV (FIG. 19C) in proportion to the RF power.
However, when a high frequency power having a relatively high frequency of, for example, about 13 MHz is used for attracting ions, if a RF power is varied, an ion energy distribution characteristics is varied as shown in FIG. 20A (low power), FIG. 20B (intermediate power), and FIG. 20C (high power). That is, while the maximum energy is varied into about 650 eV, about 1300 eV, and about 1950 eV in proportion to the RF power, the minimum energy is also varied into about 350 eV, about 700 eV, and about 1050 eV in proportion to the RF power.
Although FIGS. 19A to 19C and 20A to 20C show ion energy distribution characteristics of argon (Ar+) ions, other ions may have the same characteristics (patterns).
As described above, in the conventional method, even if the maximum energy or average energy in an ion energy distribution can be varied, the minimum energy cannot be varied independently of the maximum energy. Therefore, it is impossible to achieve an ion energy distribution characteristic indicated by, for example, a virtual line (a dashed dotted line) K in FIG. 20C. Accordingly, as also will be described in embodiments of the present disclosure, a trade-off between (an etching rate and selectivity) and an etching profile in a HARC (High Aspect Ratio Contact) plasma etching process cannot be avoided readily.